The present invention relates to a harmonic drive apparatus, and more particularly, to an improvement for strengthening and stabilizing the tooth engagement of a circular spline with a flexspline disposed within the circular spline.
In a conventional harmonic drive apparatus, such as that disclosed in U.S. Pat. No. 3,667,320, a generally elliptical wave generator is rotated by its driving shaft rotating at a high speed to rotate and deform an elliptical flexspline outside the wave generator through a bearing disposed between the wave generator and the flexspline. The flexspline has a number of external spline teeth, and a circular spline disposed outside the flexspline also has a number of internal spline teeth for locally engaging with the flexspline teeth, with the number of flexspline teeth being less than the number of circular spline teeth. The tooth engaging positions of the flexspline and the circular spline are moved in conformity with the rotation of the driving shaft. Due to the difference between the number of flexspline teeth and the number of circular spline teeth, the flexspline is rotated at a rate corresponding to the difference in the direction opposite the rotational direction of the driving shaft since the circular spline is secured to a casing of the apparatus. An output shaft connected to the flexspline on the low speed side of the apparatus is thereby rotated.
In the above conventional apparatus, the wave generator is a rigid body and the engaging states of teeth depend on only the finishing accuracy of the wave generator. Namely, when the elliptical wave generator has a length in the direction of the major axis thereof longer than a predetermined length, the bearing is excessively pressed by the wave generator and the endurance of the bearing is reduced, thereby preventing the normal movement of the bearing. On the other hand, when the wave generator has a length in the direction of its major axis shorter than a predetermined length, the flexspline teeth loosely engage with the circular spline teeth, and the accuracy of the operation of the harmonic drive apparatus is reduced. Furthermore, the apparatus portions are inevitably slightly worn away during their use for a long period, which also reduces the accuracy of their operations.